1. Field of the Invention
The present invention relates to a Raman amplifier which propagates signal light including a plurality of wavelength components through an optical waveguide and Raman-amplifies the signal light in the optical waveguide; and an optical communication system including the same.
2. Related Background Art
A Raman amplifier utilizing stimulated Raman scattering is an optical device for propagating each of pumping light and signal light through an optical waveguide, e.g., optical fiber, and Raman-amplifying the signal light when the pumping light is supplied thereto, and is employed in optical communication systems and the like. Here, wavelength components included in the pumping light are set to wavelengths shorter than those of the wavelength components included in the signal light by about several tens of nm to about 100 nm.
It has been desired for optical communication systems to suppress the occurrence of nonlinear optical phenomena since signal waveforms deteriorate due to nonlinear effects. In a wavelength division multiplexing (WDM) transmission system which transmits signal light including a plurality of wavelength components, in particular, it is important to suppress the occurrence of four-wave mixing, which is a kind of nonlinear optical phenomena, and the zero-dispersion wavelength of the optical fiber transmission line through which the signal light propagates is set to the outside of the wavelength region of the signal light in order to suppress this phenomenon.
The inventors studied conventional Raman amplifiers and, as a result, have found the following problem. Namely, even when the zero-dispersion wavelength of the optical waveguide for Raman-amplifying the propagating signal is set to the outside of the wavelength region of the signal light, there are cases where signal waveforms deteriorate due to nonlinear effects in conventional Raman amplifiers (thus lowering their transmission quality).
In order to overcome the problem mentioned above, it is an object of the present invention to provide a Raman amplifier which is hard to deteriorate signal waveforms, so that it realizes an excellent transmission quality, and an optical communication system including the same.
For achieving the above-mentioned object, the Raman amplifier according to the present invention comprises a Raman amplification optical waveguide, and a pumping light supplier. The optical waveguide propagates each of the pumping light including a plurality of wavelength components in a first wavelength band and signal light including a plurality of wavelength components in a second wavelength band, and Raman-amplifies the signal light when the pumping light is supplied thereto. The pumping light supplier supplies the pumping light to the optical waveguide.
In the Raman amplifier according to the present invention, in particular, the optical waveguide has a zero-dispersion wavelength xcex0 shifted from a center wavelength xcexcenter (=(xcexS, rms+xcexP, rms)/2) existing between a center-of-gravity wavelength xcexS, rms of the signal light and a center-of-gravity wavelength xcexP, rms of the pumping light by such a wavelength spacing that non-degenerate type four-wave mixing is effectively suppressed. The center-of-gravity wavelength xcexS, rms of signal light is given by a weighted mean of respective powers of the plurality of wavelength components included in the signal light, whereas the center-of-gravity wavelength xcexP, rms of the pumping light is given by a weighted mean of respective powers of the plurality of wavelength components included in the pumping light.
The Raman amplifier according to the present invention realizes an excellent transmission quality since the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is shifted from the above-mentioned center wavelength xcexcenter by such a wavelength spacing that non-degenerate type four-wave mixing is effectively suppressed. Optical fibers are suitable as the Raman amplification optical waveguide. The Raman amplification efficiency improves with respect to signal light in this case since the optical fibers can be made long with a low loss.
Preferably, in the Raman amplifier according to the present invention, the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is set to a shorter wavelength side than the above-mentioned center wavelength xcexcenter when the Raman amplification optical waveguide has a positive dispersion slope with respect to the signal light. Preferably, in the Raman amplifier according to the present invention, the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is set to a longer wavelength side than the above-mentioned center wavelength xcexcenter when the Raman amplification optical waveguide has a negative dispersion slope with respect to the signal light. In these cases, the occurrence of non-degenerate type four-wave mixing can be suppressed in view of self-phase modulation and cross-phase modulation as well, whereby an excellent transmission quality is obtained.
Preferably, in the Raman amplifier according to the present invention, the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is set to a longer wavelength side than the above-mentioned center wavelength xcexcenter when the optical waveguide has a positive dispersion slope d with respect to the signal light. Preferably, in this case, the Raman amplifier satisfies the following relationship:
xcex0xe2x88x92xcexcenterxe2x89xa7[48xcfx80xc2x7cxc2x7xcex3xc2x7P/(xcfx892xc2x7d)]1/3 
where c is the light velocity in vacuum, xcex3 is the nonlinear coefficient of the optical waveguide, P is the sum of respective powers of all the wavelength components included in the signal light and all the wavelength components included in the pumping light, and xcfx89 is the optical angular frequency at the center wavelength xcexcenter.
Preferably, in the Raman amplifier according to the present invention, the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is set to a shorter wavelength side than the above-mentioned center wavelength xcexcenter when the optical waveguide has a negative dispersion slope d with respect to the signal light. Preferably, in this case, the Raman amplifier satisfies the following relationship:
xcex0xe2x88x92xcexcenterxe2x89xa6[48xcfx80xc2x7cxc2x7xcex3xc2x7P/(xcfx892xc2x7d)]1/3 
In each case, the occurrence of non-degenerate type four-wave mixing can be suppressed in view of self-phase modulation and cross-phase modulation as well, whereby an excellent transmission quality is obtained. Also, the degree of freedom in designing the Raman amplification optical waveguide increases. Preferably, the zero-dispersion wavelength xcex0 of the Raman amplification optical waveguide is set to a longer wavelength side than all the wavelength components included in the signal light when the optical waveguide has a positive dispersion slope d with respect to the signal light. In this case, the occurrence of normal four-wave mixing is effectively suppressed as well.
The optical communication system according to the present invention comprises an optical fiber transmission line laid in a repeating section, and a Raman amplifier having the structure mentioned above (Raman amplifier according to the present invention). The optical communication system including the Raman amplifier can utilize, as the Raman amplification optical waveguide, an optical fiber transmission line laid in an optical repeating section of the optical communication system. Since the signal light is Raman-amplified by the Raman amplifier having the above-mentioned structure, the optical communication system can effectively suppress the occurrence of nonlinear optical phenomena, thereby yielding an excellent transmission quality.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.